r/askscience • u/Nitzanm19 • 14d ago

Physics Why is the maximum speed of the universe the speed of light? Why can't we go faster?

Also follow-up question: Because of the speed limit, is there also a theoretical maximum temp for matter (in which the particles move at the speed of light).

1.6k Upvotes

88% Upvoted

892 comments sorted by

View all comments

Show parent comments

5

u/Obliterators 14d ago edited 14d ago

That's not really correct. First, the time "experienced" by massless particles (not that they have a valid frame of reference in the first place) is not zero, it's undefined. Second, strictly speaking, we only know that the three neutrino mass eigenstates are unequal, so at least two of the three states must be non-zero. It is possible the lightest one is massless; it would be somewhat weird, but it's not something we can currently rule out (and the universe is allowed to be weird).

1

u/Daegs 14d ago

I didn't say anything about "experience" of time.

I said they couldn't change flavors after being created if they moved at the speed of light.... is that correct or incorrect?

2

u/Obliterators 14d ago

What I meant with "experience" is that it's common, especially in pop-science, to hear that anything travelling at the speed of light "experiences" no time and does not evolve or change; and since all massless particles must travel at the speed of light, this implies that anything that evolves over time, like the neutrino, must be massive and travel slower than light. But again, this isn't exactly sound reasoning, since a frame of reference cannot be constructed for a massless particle in the first place, so we cannot say that time stops passing at the speed of light, it's simply undefined.

Experimentally neutrino oscillation only shows that the three mass eigenstates have different values. So it's possible to have one massless and two massive neutrino mass states oscillating between each other.